Windows 7 Pro 64-bit for pretty much all my needs (With WinXP Mode if in the rare event I must familarize myself with the OS again)
Windows Server 2008 and Ubuntu for my experimenting non-development needs
Windows 8 Pro because $39.99 MS offer since I had Release Preview installed and if I decide to muck around in app development

Why yes I do in fact quad boot with 7 on its own HDD and the others being partitions on a 1TB drive of various sizes.

Windows XP: Was reliable, real nice thing
Vista: hahahahahahahahaahahahahaahahano
Windows 7: Very stable, very powerful, lots of convenience added
win8:tablet only, microsoft has forgotten why people like their OS, trying to be Apple when Apple is already doing a good job at that.

OSX: pretty good, it does things one way well, compatibility problems(sometimes not easily solved).

I use Arch linux on my laptop, it took some times to set it up but it's fully customized to my taste, works like a charm and boots in 15 sec even though my laptop is an old 2006 laptop bought for 200$, couldn't ask for more.

I fell in love with Linux after my Windows partition had to go and get corrupted, and have found it to do almost everything I want it to as good or better than Windows. Everything that is except for gaming, which despite taking huge strides is still sorely lacking in mainstream support. Hopefully in a couple years I will be able to ditch Windows completely, but until Linux game support becomes more universal there really isn't any other option

EDIT: forgot to mention, but Linux Mint is definitely my favorite distro. Zorin OS is also great, especially for those coming from Windows, and there are a couple others (EasyPeasy, Joli OS, Puppy) which offer some cool and useful features. There really is something for everyone when it comes to Linux, and IMO trying out different versions to find your favorite is half the fun ^_^